1. Field of the Invention
The invention relates to a process and a device for producing a printing tool. In particular, the invention relates to a process for applying a mask for sectional chemical passivation to a surface of a workpiece and to such a workpiece which has been provided with such a mask.
2. Description of Related Art
Various processes and corresponding devices are known in the prior art for producing various types of printing tools. For example, etched rotogravure forms or relief printing forms, and galvanically produced screen printing forms are known in the prior art. In the known processes for producing a printing tool, the workpiece, which in the case of a rotogravure form (for example) is formed by an unworked rotogravure cylinder, is subjected to chemical surface working. In a rotogravure form, the workpiece is subjected to surface etching which is the chemical surface working. In this known processes for producing a printing tool where the surface of the workpiece to be etched, the areas of the printing form which are not to be subjected to this surface working are covered by a mask. This mask chemically passivates the surface sections of the workpiece which are not to be worked, i.e. the covered areas of the surface are not reached by chemical surface working.
In the known processes for producing a printing tool the masks which are necessary for sectional chemical passivation of the workpieces to be worked, a photosensitive layer is applied to the surface of the workpiece, this photosensitive layer being exposed with a negative or positive of the sections of the mask to be covered or to be kept exposed and then the photosensitive layer being developed. After this development, either the exposed or unexposed areas of the photosensitive areas are flushed by washing the workpiece so that as a result the desired mask remains on the surface of the workpiece to be worked.
This process which is known from the prior art is problematic in two important respects. On the one hand, the process of applying, exposing, developing and flushing the photosensitive layer is time-consuming and labor-intensive and on the other hand, the reproducibility of the masks produced in this way is not sufficient for various purposes. It is especially problematic in reproducibility because the negative or positive for exposure of the photosensitive layer must always be attached exactly in the same position relative to the photosensitive layer. This is no longer a given in the case where the negative or positive for exposure of the photosensitive layer is placed directly on the latter when the negative or positive at each location of the photosensitive layer does not have the same distance to the latter.
The reproducibility of application of the mask is of special importance particularly in relief printing which requires high imaging accuracy. In relief printing, for three-dimensional working of the paper, surface structures with relatively large and varied height differences are produced on the printing form. To do this, the printing form is etched repeatedly and is also covered repeatedly by a mask. With the known process for producing a printing form, the masks which are applied in succession cannot be positioned accurately enough relative to one another to guarantee the desired imaging accuracy.
Moreover, in the known processes, it is still problematic that flexibility is low in mask production. For each new mask, a new negative or positive must be produced for exposure of the photosensitive layer which in turn entails considerable cost.
In view of the foregoing, it is a primary object of the present invention to make available a process and a device for producing a printing tool which makes it possible to quickly, reproducibly and variably apply a mask for chemical passivation to the surface of a workpiece to be worked.
The pximary object above is achieved in accordance with the present invention in that the mask is produced from wax and the wax is sprayed section-by-section onto the workpiece from several nozzles in spots with EDP support, i.e., using an electronic data processor control. Therefore, in accordance with the present invention, the wax mask is applied in spots on the surface of the workpiece in a manner somewhat similar to known inkjet printers. With the process in accordance with the present invention, printing tools which may be in any shape including printing plates and printing cylinders, can be worked.
Since waxes are resistant to most acids and moreover, have low electrical conductivity and are at the same time water-repellant, they are suitable for chemical passivation of surfaces both in surface etching, as well as in galvanic surface coating. The EDP-supported spraying of wax onto the surface of the workpiece with the currently available drive engineering for the printing heads necessary for this purpose enables very high reproducibility in the production of the mask and maximum possible flexibility since any shape of the mask stored in the EDP system can be chosen and sprayed on via EDP-supported spraying. The present invention is also advantageous in that the mask can be applied to the surface of the workpiece to be worked in a single process.
A first advantageous embodiment of the process in accordance with the present invention is the use of an etching technique for the chemical surface working. As already mentioned, waxes are resistant to most of the acids used in surface working by etching. In this way, during etching, only the sections of the surface of the workpiece which are not covered by the wax are worked.
Because a printing form for rotogravure printing or relief printing is worked as the workpiece, and at the same time an etching technique is used, the printing forms for rotogravure or relief printing can be produced quickly, reproducibly and variably by the process in accordance with the present invention. In particular, by practicing the present invention in the production of printing forms for relief printing, it is guaranteed that even complex height profiles which are produced in several etching steps using different masks can be produced with the desired imaging accuracy.
If a mold for galvanic coating for producing a printing tool is worked as the workpiece, screen printing forms can be produced with the process in accordance with the present invention.
The process in accordance with the present invention is made especially advantageous by a special procedure for spraying wax onto the workpiece, in which the nozzles are arranged next to one another in the direction of the X-axis, the nozzles and the workpiece move in the direction of the Y-axis relative to one another, the nozzles are combined functionally into groups of at least one nozzle at a time, the nozzles and the workpiece are moved relative to one another in the direction of the X-axis after working of the tool in the direction of the Y-axis to a new position with reference to the X-axis, and the step width of the motion in the direction of the X-axis corresponds to the group width minus or plus the distance of adjacent nozzles divided by the number of groups. According to this embodiment of the process in accordance with the present invention, the wax layer is applied to the workpiece by the printing head, for example, during rotation of a cylindrical workpiece. In this regard, the printing head which has the nozzles is advanced after each revolution of the workpiece far enough so that the group bordering the group preceding applies the bordering wax points directly next to the wax points applied by the previous group during the previous revolution. This means that for each revolution of the workpiece, it is not the complete mask which is applied to the workpiece in the area of the nozzles, but only a fraction of the mask, while the next fractions are then applied by the following groups.
This embodiment of the process in accordance with the present invention differs, for example, from the known process for producing an inkjet print image. The advantage of the described embodiment is that the distance between the nozzles can be chosen to be much larger than required if the complete graphics would be applied with maximum resolution as the printing head moves past the workpiece.
This described embodiment of the process in accordance with the present invention is further improved by the fact that the group which is last in the direction towards the motion along the X-axis is formed from one nozzle less than the remaining groups. Because the last nozzle of the last group is eliminated, it is ensured that a wax point is not sprayed twice onto the surface of the workpiece to be worked. In this way the resolution of the wax layer applied to the workpiece is made uniform.
A device which is known from the prior art for producing a printing tool with a receiver for a workpiece and a means for applying a mask for sectional chemical passivation of the surface of the workpiece in accordance with the present invention, is developed especially for implementation of the above described process where the mask is essentially of wax. The means for applying the mask has EDP control and the means for applying a mask has at least one printing head with several nozzles for spot spraying of the wax onto the workpiece. This device in accordance with the present invention guarantees implementation of the advantages described with respect to the process as described previously.